The present invention relates to a method of manufacturing a stamper for information storage mediums in the form of discs.
Various recording and playback systems have been developed to the point of practical use which utilize information storage mediums in the form of discs containing video and/or audio information recorded as depressions or pits at a high density. Such known discs are made of a copolymer of vinyl chloride and vinyl acetate by a casting process, an embossing process, or a compression molding process, as with ordinary disc records. Alternatively, radiation curing resin such as resin hardenable by exposure to light or an electron beam has been used to form replicas copied from a stamper. Whatever the process, metal stampers are employed to stamp finished discs.
The metal stampers are manufactured according to the following process: A photoresist layer is formed on a surface of a disc of glass which has a high degree of flatness, and then is irradiated with a laser beam or electron beam modulated in intensity by information signals to be recorded. The photoresist layer is then developed to form depressions or pits corresponding to the information signals. A thin film of silver is deposited by chemical plating on the surface of the photoresist layer in a step known as silver mirror treatment to render the photoresist layer surface electrically conductive. Thereafter, a layer of nickel is deposited on the silver film by electroplating, and is separated therefrom, the separated layer of nickel serving as a metal master. Another layer of nickel is deposited on the surface of the metal master by electroplating, followed by separation of the deposited nickel layer as a metal mother from the metal master. A metal stamper is prepared by depositing a layer of nickel on the metal mother by electroplating and detaching the deposited layer of nickel, which can be used as the metal stamper.
The prior metal stamper is therefore fabricated through a number of steps by repeating chemical plating, electroplating, and electroforming processes several times. Such processes, however, produce a large quantity of waste materials such as plating solutions which are a cause of public hazard or environmental pollution.
Discs are produced from the metal stamper thus manufactured in a compression molding process, for example, a disc material is heated and pressed by the stamper on which a pressure ranging from 100 to 150 Kg/cm.sup.2 is imposed. On the other hand, the stamper is subjected to a much smaller pressure of only a few hundreds g/cm.sup.2 when the discs are to be made of radiation curing resin. Accordingly, it is reasonable to conclude that stampers used for stamping discs formed of radiation curing resin are not required to be as mechanically strong as the metal stampers conventionally used for pressing discs as in the compressing molding process.